Casting hydrides

ABSTRACT

A method of casting lithium hydride, lithium deuteride or lithium tritide, or mixtures of these with zirconium hydride in a titanium or titanium-coated mold under a hydrogen atmosphere exceeding the dissociation pressure of titanium hydride, whereby, on cooling, the cast material can be readily removed from the mold without damage thereto.

United States Patent Phillips, Jr.

[451 Sept. 19, 1972 [S4] CASTING HYDRIDES [72] Inventor: Ernest C.Phillips, Jr., 2533 Albeworle Court, Richland, Wash. 99552 [22] Filed:July 2,1969

[21] AppLNos 838,717

[52] US. Cl ..264/.5 R, 264/85, 264/332,

252/301.1 R, 23/204, 23/360, 117/53 [51] Int. Cl. ..G2lc 21/00 [58]Field of Search ..264/85, 299, 332, 319, .5;

23/204, 360; 1l7/S.l, 5.3; 65/355; 252/301.1

[56] References Cited UNITED STATES PATENTS 3,023,119 2/1962 Anderson eta1. ..........117/5.1

3,126,294 3/1964 Pichler ..117/5.3 3,224,860 12/1965 Stinnes ..65/355Primary Examiner-Carl D. Quarforth Assistant Examiner-Stephen J.Lechert, Jr. Attorney-Roland A. Anderson [5 7] ABSTRACT 6 Claims, NoDrawings CASTING rrvmunas BACKGROUND OF THE INVENTION hydrides,particularly to casting of lithium hydride, lithium deuteride andlithium tritide, and more particularly to the casting of such materialsin titanium or titanium-coated molds.

Prior to this invention, lithium hydride, deuteride, or tritide, whichmaterial is utilized, for example, as moderators for nuclear reactorsand neutron shields, has been cast into iron, steel, or stainless steelmolds to which the cast material adhered tenaciously. Removal of thecast material from such molds damages the surface of the material andoften causes fractures in the material. Furthermore a thin layer of thecast material usually remains on the surface of the mold, thus requiringremoval of this thin layer prior to subsequent reuse of the mold. Due tothe surface damage of the cast material by the prior known molds,further finishing procedures have been required, thus increasing thecosts.

While mold release coatings are known in the prior art, none of thisprior known coatings have been found satisfactory for the casting oflithium hydride, deuteride or tritide.

SUMMARY OF THE INVENTION The process of this invention over comes theproblems of the prior art in the casting of lithium hydride, deuteride,or tritide by eliminating adherence between the mold and cast material,thereby producing a casting without damage to the surface thereof orfractures therein since the casting is not placed under stress by themold during cooling and is readily removed from the mold. In addition,utilization of the present inventive casting method leaves the mold freeof adhered casting material, thereby reducing the time and expense inthe casting operation.

Therefore, it is an object of this invention to provide an improvedmethod for casting lithium hydrides, deuteride and tritides.

Another object of the invention is to provide an improved casting methodwhereby the casted material is easily removed from the mold withoutdamage to the cast material.

Other objects the invention will become readily apparent from thefollowing description of the improved casting process.

DESCRIPTION OF THE INVENTION As set forth above, lithium hydride,lithium deuteride, or lithium tritide, or mixtures of these withzirconium hydride has found numerous uses, especially in the nuclearfield, as moderators for nuclear reactors and neutron shields. For suchuses, castings of these materials in various desired shapes and withdesired surface finishes are required. Such castings are readilyproduced in accordance with this invention by the utilization oftitanium molds or molds of metals such as iron, steel or stainless steelwhich have been coated with titanium and casted as describedhereinbelow.

During the casting process and during the cooling cycle, the mold andlithium hydride, for example, are

kept under a hydrogen atmosphere, the pressure of which exceeds thedissociation pressure of titanium 5 hydride. The time at temperatureunder the hydrogen atmosphere must be sufficient for the titanium tohydride and is, therefore, dependent upon the thickness of the titaniummold or titanium coating. Consequently, it is desirable to use thethinnest titanium practical. If sufficient time at temperature is notallowed, the titanium will acquire hydrogen, deuterium, or tritium fromthe cast material, leaving a small layer of metallic lithium between themold and the casting, making it difficult to separate the casting fromthe mold.

During the casting process, usually at temperatures of 1,300" to l,400F, the titanium of the mold is partially hydrided. The cast lithiummaterial has a greater coefficient of thermal expansion than the moldmaterial (titanium) and does not adhere to the mold. During cooling, thecast lithium material shrinks more than the mold and separates from themold. When cold, the mold and casting are easily separated. Since thecast lithium material does not adhere to the mold, the casting is notplaced under stress by the mold during cooling, therefore, eliminatingdamage to the casting due to the mold. However, the surface finish ofthe casting is dependent upon the finish of the titanium surface of themold.

By way of example only, the following operational sequence is set forthto provide a better understanding of the inventive casting method.

l. Molten lithium hydride, deuteride or tritide is poured into atitanium or titanium-lined mold.

2. The mold is placed in a furnace having a temperature maintained atapproximately l,300 to 1,400 F and in a hydrogen atmosphere exceedingthe dissociation pressure of titanium hydride, the cast material havinga greater coefficient of thermal expansion than the mold and thus doesnot adhere to the mold.

3. The furnace temperature and hydrogen pressure are maintained for aperiod of time sufficiently long to convert the titanium to titaniumhydride, the time being dependent on the thickness of the titaniumutilized.

4. The mold is cooled while maintaining the hydrogen atmosphere pressureto prevent dissociation of the titanium hydride wherein the castingshrinks more than the mold and does not place any stress on the castingduring cooling.

5. After cooling, removal of the casting from the mold is facilitated bythe shrinking of the casting which separates samefrom the mold. Sincethere is no adherence between the mold and the casting, the surface ofthe casting is not damaged and the casting is not placed under anystress during the cooling, thereby eliminating damage to both thecasting and the mold.

The following examples illustrate tests carried out utilizing thehereinbefore described inventive method:

EXAMPLE I A piece of titanium tubing was used as a mold. During thecasting an atmosphere of hydrogen at a pressure of about 860 mm Hgabsolute was maintained. Lithium hydride was cast and, after cooling,was readily removed from the mold. In tests performed as directly aboutbut with insufficient hydrogen present there was severe sticking of thecasting to the mold, and the hydride cracked on cooling due to theadherence of the periphery thereof to the mold and to the resultingtensile stresses.

EXAMPLE II A mild steel mold was coated with a very thin layer oftitanium by evaporation in vacuum. Again, lithium hydride wassuccessfully cast under an atmosphere of hydrogen (same pressure) andeasily removed from the mold. In similar tests with a mild steel mold,it was found that when either the titanium coating or the hydrogenatmosphere was omitted, sticking occurred.

EXAMPLE III A mold of stainless steel was coated and used in Example llabove with the same results. Tests with the stainless steel mold showedthat sticking occurred as with the mild steel mold when either thetitanium coating or the hydrogen atmosphere was omitted.

Although only lithium hydride, deuteride or tritide have beenspecifically considered in the description of inventive method, it isbelieved that calcium hydride could be cast by this process. While highmelting hydrides, such as zirconium hydride would probably not castsatisfactorily; a mixture of lithium hydride, deuteride or tritide withzirconium hydride way be cast in accordance with the invention.

While the titanium or titanium-coated mold may be pre-treated withhydrogen to form a hydrided surface before carrying out the castingoperation, this is not necessary since the hydrogen diffuses so rapidlythrough the lithium hydride, for example, and apparently even along verythin coatings of titanium, that it performs its intended function evenwhen the mold is filled with lithium hydride.

It has been shown that the present invention provides an improved methodfor casting materials and particularly for casting lithium hydride,deuteride or tritide.

Although specific parameters for the inventive castcations as comewithin the spirit and scope of the invention.

lclaim:

l. A method of casting materials comprising the steps of: containing acasting material selected from the group consisting of lithium hydride,lithium deuteride, and lithium tritide in a mold at least lined with alayer of titanium, heating the thus contained casting material to atemperature so that the titanium of the mold is at least partiallyhydrided and in a hydrogen atmosphere having a pressure greater than thedissociation pressure for a time period sufficiently long to convert thetitanium to titanium hydride, cooling the thus cast material whilemaintaining the hydrogen pressure to prevent dissociation of thetitanium hydride, and removing the casting from the mold.

2. The method defined in claim 1, additionally inclu in the ste ofcoatin the la er of titanium 0 a mo d ormed of metal seleted froih thegroup consi sting of iron, mild steel and stainless steel.

3. The method defined in claim 1, wherein the heating temperature ismaintained in the range of about l,300 to l,400 F.

4. The method defined in claim 1, wherein the hydrogen atmosphere ismaintained at a pressure of about 860 mm Hg absolute.

5. The method defined in claim 1, wherein the material to be cast fromthe selected group is mixed with zirconium hydride prior to the step ofcontaining the casting material in the titanium lined mold.

6. The method defined in claim 1, additionally including the step ofpre-treating the titanium lined mold with hydrogen forming a hydridedsurface thereon prior to containing and heating the casting materialtherein.

2. The method defined in claim 1, additionally including the step ofcoating the layer of titanium on a mold formed of metal selected fromthe group consisting of iron, mild steel and stainless steel.
 3. Themethod defined in claim 1, wherein the heating temperature is maintainedin the range of about 1,300* to 1,400* F.
 4. The method defined in claim1, wherein the hydrogen atmosphere is maintained at a pressure of about860 mm Hg absolute.
 5. The method defined in claim 1, wherein thematerial to be cast from the selected group is mixed with zirconiumhydride prior to the step of containing the casting material in thetitanium lined mold.
 6. The method defined in claim 1, additionallyincluding the step of pre-treating the titanium lined mold with hydrogenforming a hydrided surface thereon prior to containing and heating thecasting material therein.